Modeling mechanical layering effects on stability of underground openings in jointed sedimentary rocks

This paper examines the significance of mechanical layering for “blocky” rock mass deformation around underground openings excavated through sedimentary rocks. The analysis is based on an integration of geologically based discrete fracture models (“geoDFN”), which incorporate “mechanical layering”, with the numerical discrete element method—the discontinuous deformation analysis (DDA). We begin with addressing limitations of classical solutions for mine roof stability in layered and jointed rock masses via the analysis of the free standing, unsupported, 2000-year-old underground quarry known as Zedekiah's cave below the old city of Jerusalem, Israel. We show that both the “clamped beam” model and the “Voussoir beam analogue” fail to predict the observed roof stability. Only application of discrete element modeling, which allows for interactions between multiple blocks in the rock mass, can capture correctly the arching mechanism which takes place in the roof and which properly explains the long-term stability of this underground opening.We continue with examining the effect of joint trace geometries on “blocky” rock mass deformation using the hybrid geoDFN-DDA approach. We show that with increasing joint length and decreasing bridge length vertical deformations in the rock mass are enhanced. We explain this by the greater number of distinct blocks in the rock mass due to the greater joint intersection probability in such geometries. We find that rock bridge length is particularly important when considering the stability of the immediate roof. With increasing rock bridge length the number of blocks in immediate roof decreases and consequently individual block width is increased. Increased block width in immediate roof layers enhances stable arching development, thus improving their load carrying capacity and overall stability of the underground structure.